The purpose of this research is to characterize how the physiological ligands of the sodium pump regulate active cation transport in human red blood cells. Current ideas on the reaction pathway by which the cardiac glycoside-sensitive Na pump catalyzes the exchange of Na and K are derived from experimental data from two types of sources, enzymatic studies where vectorial and transport properties are lost and transport studies in intact systems e.g. red cells, where measurement of enzymatic conversions has been difficult. In the present work, resealed human red cell ghosts will be employed where independent control of intracellular and extracellular compartments is possible. Na pump-mediated processes where ATP or P/i are the substrates will be initiated using photorelease from intracellular caged ATP or caged P/i. In this way, the effects of membrane potential on the ATP:ADP exchange rate will be measured, the effects of various ligands on phosphorylation rates and levels as well as the relative rates of ATP:ADP exchange and Na transport. The basis and consequences of the anomalous temperature dependence of phosphoenzyme distribution of the red cell Na pump will also be examined as well as the transport properties of the red cell Na pump in the absence of the normal physiological ligands. The overall aim is to determine to what extent existing models of the Na pump account for the physiological functioning of Na:K exchange. Recent experimental evidence suggests an abnormal working of the red cell Na pump in a variety of disease states, e.g. hypertension, obesity, muscular dystrophy, sickle cell anemia. A more detailed understanding of the functioning of the normal Na pump is required before these observations can be properly evaluated in disease states.